The Shield left half is the left half of the dragon sq shield and is obtained as a very rare drop from many monsters around RuneScape. With 60 Smithing, it can be combined with a "Shield right half" (Bought in the Legend's Guild for 750k coins) to create the dragon square shield, also yielding 75 Smithing XP.

Trivia

The term "left half" is a misnomer- The half appears to be the top half of the shield, not the left half.